1. Field of the Invention
The invention generally relates to a liner used for a high pressure gas container, and also relates to a high pressure gas container having the liner. In particular, the invention is concerned with a liner for a high pressure gas container, which has high degrees of gas impermeability, pressure resistance, and impact resistance.
2. Description of Related Art
Automobiles that use compressed natural gas (hereinafter referred to as “CNG”) as a fuel for engines are equipped with high pressure gas containers that are charged with the CNG.
FIG. 1 illustrates an example of high pressure gas container 7, which principally consists of a cylindrical liner 1 for a high pressure gas container, and a shell 2 disposed on the outer surface of the liner 1. A gas chamber 10 is defined by the liner 1 within the shell 2. In addition, bosses 3 are provided at opposite ends of the liner 1, and an opening 30 is formed through a central portion of each of the bosses 3. A high pressure gas, such as CNG, is supplied to or discharged from the gas chamber 10 of the container 7, through the opening 30.
A typical example of the liner for the high pressure container is made of a metal, such as steel, as disclosed in, for example, U.S. Pat. No. 4,714,094. However, the liner made of a metal suffers from a relatively heavy weight. If the high pressure container having such a heavy liner is mounted on a motor vehicle, the vehicle weight becomes heavier, resulting in reduced fuel economy. In view of this problem, light-weight liners made of a resin have been widely used in place of the liners made of a metal. The resin liners may be formed of, for example, polyamide, polyethylene, or the like, as disclosed in U.S. Pat. No. 5,025,943.
It is, however, noted that the liner made of polyethylene has relatively high gas permeability. During use of the container, therefore, hydrocarbon gases may be permeated through the liner, to be discharged or released to the atmosphere, which is undesirable in terms of environmental protection. On the other hand, the liner made of polyamide has excellent gas impermeability as compared with the liner made of polyethylene, but has relatively low impact resistance and chemical resistance. Although the impact resistance may be improved by adding a plasticizer to polyamide, the resulting liner is more likely to allow gas to permeate therethrough. It is thus difficult to provide both excellent gas impermeability and excellent impact resistance at the same time. In addition, the liner for a high pressure container is required to exhibit high pressure resistance, since it is normally exposed to a high pressure during use.
The liner having high degrees of gas impermeability, pressure resistance and impact resistance as described above is also desired to be used in a container that is charged with a gas, such as methane, hydrogen, oxygen, or nitrogen, at a high pressure, as well as the CNG container as indicated above.